1. Field of the Invention
The present invention relates to a rubber composition for a tire, which is excellent in grip performance on a wet road (hereinafter referred to as "wet grip") while providing fuel savings. The present invention is also directed to a tire tread formed of such a rubber composition, and to a tire having such a tire tread.
2. Description of the Related Art
Recently the fuel saving performance of automobiles has been increasingly desired, and since the fuel saving performance depends significantly on tire characteristics, rubber compositions for tires which provide for excellent fuel saving performance are always under development. On the other hand, the wet grip performance of tires has been of great interest from the view point of safety driving.
Generally, a tire is required to have a reduced hysteresis loss so as to exhibit satisfactory fuel saving. However, a reduced hysteresis loss leads to a reduced rolling resistance and, therefore, the fuel saving results in reduced grip performance. On the other hand, a tire of higher wet grip performance naturally has a higher rolling resistance, which results in an increased hysteresis loss, hence a low fuel saving performance. Thus, the wet grip performance and fuel saving performance of a tire are not compatible with each other, and thus it is difficult to obtain a tire satisfactorily exhibiting both of these features. Therefore, an intensive study has been made to develop a rubber composition which provides a tire satisfactorily exhibiting both the wet grip performance and the fuel saving performance.
For example, it has been conventionally known that the wet grip performance is improved by using epoxidized natural rubber for a tire tread. In this case, the fuel saving performance is adversely affected, because the rubber cannot escape from the non-compatible relationship between the two characteristics. In addition, such a rubber shows a strong rolling adhesion and hence tends to offer poor rolling workability. To compensate for this adverse effect, use of silica instead of carbon black has been proposed (RUBBER WORLD, by C. S. L. Baker, et al., August 1987, p. 27 and ELASTOMERICS, by Roland Newell, July 1992, p. 22). This technique still involves disadvantages in workability, wear resistance and the like.
Alternatively, there has been proposed a rubber composition in which a compound having at least two primary amino groups and a sulfur vulcanizer are mixed with synthetic cis-1,4-polyisoprene rubber having an epoxy group in order to improve the tensile stress and strength of vulcanized synthetic cis-1,4-polyisoprene (Japanese Examined Patent Publication No. Hei 1(1989)-24166). With this rubber composition, however, both the wet grip performance and the fuel saving performance cannot be rendered compatible.
The present invention has been accomplished in view of the above mentioned technical background.
It is therefore an object of the present invention to provide a rubber composition which provides for a tire satisfactorily exhibiting both fuel saving and wet grip performances.
The present invention has intensively studied rubber compositions comprising a diene rubber having an epoxy group or a mixture of a diene rubber having an epoxy group and a diene rubber free of any epoxy group, a diamine compound having a primary amino group, and carbon black, and found that both the wet grip performance and the fuel saving performance are improved when tan.delta. at -10.degree. C. and tan.delta. at -70.degree. C. satisfy specific requirements. Thus the present invention has been accomplished based on such a finding.